When a NiCd battery or NiMH battery is fully charged, any additional charge current is converted to heat within the cell and needs to be dissipated in order to reduce the damaging effects of high temperature on the battery's service life. The most common method found in many low cost chargers is to inject current into the battery(s) at a very low rate. The benefit of this method is that once the battery is fully charged, the minimal amount of heat generated by the continuous overcharge current can be adequately dissipated through the cell walls and the cell's temperature is not high enough to cause serious damage to the battery. The adverse affect of this method is that it takes many hours (e.g. 20) to recharge the cells to 100% state of charge (SOC). Another problem is that in some climates, the self-discharge rate may be higher than the charge rate from the charger. This results in a no-charging condition or where the battery(s) charges to a finite percentage below 100% SOC or even discharges to zero % SOC.
To cause a faster recharge time, some “fast” chargers are available which inject higher current, but the charge period is terminated by either user intervention (as required by user manuals, quoting “before permanent damage to the battery(s) is incurred.”) or by a timer measuring a fixed time interval from the start of charge. In either case, more times than not, the battery(s) is either left charging too long or too short resulting in damaged or undercharged battery(s).
A more sophisticated approach used by higher cost systems is to inject charge at a higher rate, but stop the high-rate charge automatically after an end-of-charge indication is sensed. In NiMH and NiCd batteries, as the 100% SOC charge condition is approached, the temperature and terminal voltage rise rapidly. Therefore, the end-of-charge condition can be sensed by the charger measuring the battery(s) terminal voltage(s), the rate of change of terminal voltage or temperature levels. However, voltage and temperature levels and their rates of change are determined by not only the battery's state of charge but the ambient temperature and charge rates as well. The higher the rate of charge and ambient temperature, the higher the battery's case temperature and its terminal voltage will be at any state of charge. It becomes a daunting task to program a smart charger to take into account all of the environmental and forcing functions to determine the proper 100% SOC point and terminate fast charge for optimum battery performance.
The use of UPS systems having battery backup systems to provide regulated, uninterruptible power for such equipment as computer systems is well known. Typically, most UPS systems use some type of lead acid battery to provide backup power. Lead batteries, however, have performance limitations especially when they are discharged at rates well above their specification rates or when they are operated at temperature extremes. NiMH and NiCd battery chemistries provide advantages when used in UPS systems, detailed below.
While the invention is disclosed as being useful in the charging of NiMH and NiCd batteries in a UPS system, it is understood that it may be used in many other environments, for example, as a standalone charging device or used in other equipment and devices which contain such batteries that would periodically require recharging.